Explain issue related to L7 load balancers with apm-server

[rubvs]

Description

Add explanation of potential issues that can arise when using an L7 load balancer with APM Server.

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

Closes elastic/apm-server#13741.

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}

[github-actions]

Warning

It looks like this PR modifies one or more .asciidoc files. These files are being migrated to Markdown, and any changes merged now will be lost. See the migration guide for details.

[mergify]

This pull request does not have a backport label. Could you fix it @rubvs? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[carsonip]

thanks! You've covered it well. I've amended the descriptions to try to make them clearer. Lmk your thoughts.

[carsonip]

Also, docs are migrated to a new repo. I believe all 9.0 docs should go to https://github.com/elastic/docs-content , which is also why you're seeing the asciidoc warning

[rubvs]

@carsonip thanks, I've created a PR in the new docs: elastic/docs-content#614. Should I close this one?

[carsonip]

Superseded by elastic/docs-content#614